The frequency causes the fringing pattern to change due to change in frequency, The RLG determines the angle by counting pulses from the photodetector. 2/23/2015 ELEC 4504 Avionics Engineering 94 Inertial Navigation – Gyroscopes Ring Laser Gyro Problems: •Lock-in at low rotation rates due to weak coupling between the two resonant systems (coupling due to mirror backscatter) Analagous to static friction (stiction) in mechanical systems Causes a dead zone Alleviated by “dithering” the gyro at a few hundred Hz •Random loss of pulses at the output ( causes “drift”)
2/23/2015 48 2/23/2015 ELEC 4504 Avionics Engineering 95 RLG Note: The sensitivity (resolution) of Ring Laser Gyro is: N=4A/λL Where: N is the number of fringes per radian A is the area enclosed by the path L is the Length of the path λ is the wave length of the light Note that the larger the area, the more sensitive the gyro 2/23/2015 ELEC 4504 Avionics Engineering 96 Example: RLG with an equilateral triangular path of length 21cm and wavelength 980nm - sides are 7 cm each - A=(b*h)/2 =3.5*7*.866 =21.2cm 2 - N=4A/λL = 4*21.2x10 -4 /(980x10 -9 x21x10 -2 )=41205 Fringes per radian or 719 fringes per degree Approximately 0.0014 degrees per fringe RLG
2/23/2015 49 2/23/2015 ELEC 4504 Avionics Engineering 97 Inertial Navigation – Gyroscopes Fibre Optic Gyro Similar concept to RLG except that amplification is not used Two strands of optical fibre are wound in opposite directions on a coil Laser light is sent from a single source down both fibres The outputs of the two fibres are combined at a photodiode Rotation of the coil around its axis causes the two paths to have different lengths and the output of the photodiode provides a light dark pattern. Each cycle indicates an increment of angular rotation 2/23/2015 ELEC 4504 Avionics Engineering 98 Inertial Navigation – Gyroscopes Fibre Optic Gyro Has the advantage of being rugged and relatively cheap Sensitivity increases with length of fibre Unfortunately, the longer the fibre, the lower the output signal. Used on low performance systems
2/23/2015 50 2/23/2015 ELEC 4504 Avionics Engineering 99 Inertial Navigation – Gyroscopes MEMS Gyro All gyros to date have been quite large in fact the sensitivity of spinning mass gyros and RLGs are a direct function of their size. Efforts are being made to apply MEMS technology to gyros as well as to accelerometers MEMS gyros exhibit high drift rates and thus are not suitable for commercial aviation use They are used in conjunction with GPS in “coupled” systems which use the best characteristics of each MEMS gyro: 2/23/2015 ELEC 4504 Avionics Engineering 100 Inertial Navigation – Strapdown Systems The main problem for an INS is to separate the vehicle acceleration from the effect of gravity on the accelerometers In the stable platform, this is done by maintaining the accelerometers perpendicular to the gravity vector which allows us to ignore the effect of gravity Another approach is to keep track of the gravity vector and subtract its effect from the outputs of the accelerometers This is an analytical or computational implementation. Made feasible with advancements in microprocessors
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